1. Field of the Invention
The present invention relates generally to a liquid crystal display device and a method of driving the liquid crystal display device, and more particularly to a liquid crystal display device including an OCB liquid crystal and a method of driving this liquid crystal display device.
2. Description of the Related Art
In general, a liquid crystal display device includes a liquid crystal display panel which includes a pair of substrates and a liquid crystal layer held between the pair of substrates; a surface light source device which illuminates the liquid crystal display panel; and a control unit which controls the liquid crystal display panel and the surface light source device. The liquid crystal display panel has a display section composed of a plurality of display pixels which are arrayed in a matrix. Further, a plurality of source lines are disposed along the columns of the display pixels, and a plurality of gate lines are disposed along the rows of the display pixels. In each display pixel, a pixel switch is disposed near an intersection of the associated source line and gate line.
In the case of driving the above-described liquid crystal display device, a state in which an image is displayed is retained during a 1-frame period by the pixel switch of each display pixel. Thus, compared to a display device such as a cathode-ray tube (CRT), it is difficult to improve the visibility of a moving image.
In order to improve the moving image visibility, for example, in an OCB mode liquid crystal display device, the feature that the responsivity of the OCB mode is very high is made use of, and it has been proposed to perform a black insertion driving scheme in which a period for video display and a period for non-video display are cyclically provided in every 1-frame period (see, e.g., Japanese Patent Applications No. 2000-214827 and No. 2002-107695).
In the above-described OCB mode liquid crystal display device, when the black insertion driving scheme is executed, in order to obtain high contrast, the non-video signal needs to be adjusted to have a minimum-transmittance voltage that is optimal for black display. Thus, in a color-display-type liquid crystal display device, non-video signals need to be independently adjusted so that the non-video signals may have optimal pixel voltages for black display in association with the respective display pixels.
In addition, the non-video signal needs to be set at a voltage or more, at which reverse transition of the OCB liquid crystal (phase transition from a bend alignment state to a splay alignment state) does not occur. This voltage influences the black insertion ratio and the white display voltage.
In general, the optimal pixel voltage for black display is set at a single value for reasons of optical device design of the OCB mode liquid crystal display device. Thus, in a case where the non-video signal set at the optimal value for black display is not at a threshold or more for preventing reverse transition of the OCB liquid crystal, the black insertion ratio or the white display voltage is adjusted so as to make the voltage, at which reverse transition occurs, lower than the optimal voltage for black display.
However, such problems may arise, in some cases, that the contrast and luminance of a display image deteriorate, due to the setting of the non-video signal at the optimal value for black display and at the voltage which can prevent the reverse transition of the OCB liquid crystal, as described above.